The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a siliconp‐type photon diode and a 0.14 cm3ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0° (parallel to the detector stem) to a direction transverse to the detector stem (90°), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%–22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter ≤4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.